The present invention relates to a method and to apparatus for monitoring and controlling production line filling of receptacles with a predetermined weight of a variable density material.
In many industries, e.g. in food, oil, pharmaceuticals, or chemistry, there is a need to package liquid or pasty materials which are of variable density. In homogeneous materials, such density variation may simply be due to temperature expansion or contraction, while in some heterogeneous materials density variation is a specific feature of the material. In either case, there is a problem when a production line of receptacles is to be filled with a predetermined fixed weight of the material.
If conventional fixed volume filler apparatus is used, it is necessary to set the filling apparatus to fill the receptacles with too much material as a general rule, so that the minimum weight is guaranteed even under worst case conditions, i.e. when the material is at its lowest density.
Such systematic overfilling of the receptacles to guarantee a minimum weight represents a direct loss to the manufacturer of the material, since the filling errors are cumulative and not in the manufacturer's favor. The cumulative overfill of un-payed-for material shipped to the customer may be far from negligible.
It is also possible to use filling installations that use an inbuilt balance to produce a constant weight fill by weighing each receptacle individually. This does at least eliminate problems due to varying density. However, in practice there are several difficulties. Firstly, accurate weighing requires a certain minimum settling time. Unfortunately this settling time is longer than the time needed to fill the receptacle, particularly on high throughput filling installations such as are used for filling jam jars, for example. (Since jam jars have a wide opening they can be filled very fast). Additionally, some receptacles, particularly those made of glass, do not all weigh exactly the same as one another, so the weighing operation must be performed twice for each receptacle, once when empty to determine the weight of the receptacle, and then again when full to measure the weight of the material with which it has been filled. This requires at least two settling periods. Clearly such double weighing could be avoided by systematically overfilling to make allowance for the worst expected variation in receptacle weight, but that negates the advantage which weighing is supposed to bring over volumetric filling. Thus, filling installations that weigh the material either require the throughput of the production line to be reduced to less than the throughput which can be achieved by volumetric filling, or else the throughput can be kept up, but only at the expense of the weighing operation becoming somewhat approximate in which case it is again necessary to overfill the receptacles by enough to make up for expected errors of approximation. Thus, presently available filler installations which include a weighing step are not completely satisfactory since they lead either to a loss of time or else to a loss of material--and in either case that means a loss of income for the manufacturer.
Preferred implementations of the present invention mitigate these drawbacks.